On tower computers made in the past 8-10 years, the speaker socket is colour-coded so you can easily find the right socket (this colour coding system is known as AC'97).
The speakers go in the light green socket, and there will usually be two others, one is light blue for secondary sound output, and the other is pink, used for microphone input.
Computers with older sound systems may have all black sockets instead, invented before the AC'97 system of colour coding, in which case there will usually be a very small picture indicating which is the speakers.
The easiest way with older black socket designs is to start the PC playing music, and then go thru each socket (it's perfectly safe to plug speakers into the wrong one of the available sockets - none of these things carry high voltage of any kind.
High specification modern towers may have 6 or more sockets available, as these are needed for surround sound Dolby 7.1 speaker outputs. With that type, they are split into 2 groups of 3, with 1 group covering the standard pink, green and blue sockets, and then the extra ones next to them.
You may also see a small square audio socket on your PC if its top of the range, and this may have a red glow from it. This is fibre-optic audio output, and you should avoid ever looking directly into these illuminated sockets as they are like a laser light inside.